<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>  
  <div id="box">
      盒子 
  </div>
  <div id="con"></div>
  <div class="cc">123</div>
  <div class="cc">dd</div>
  <div class="cc">gg</div>
  <button id="btn">点击</button>
  <script src="https://cdn.jsdelivr.net/npm/jquery@3.2/dist/jquery.min.js"></script>
  <script src="./index.js"></script>

</body> 
</html>